Constant-intensity-light sources are ubiquitous in optical sensing systems used in medical, environmental, and industrial applications. Temperature stability of these optical sources is essential as it is the foundation of the instrument accuracy, be it through calibration at manufacturing time or in real time correction. Optical and electrical properties of all light emitting components and optical materials vary with temperature. For this reason an absolutely stable source does not exist. Rather, sources of varying stability are developed for specific applications. In many applications today, and specifically, in turbidimeters and nephelometers, light-emitting diodes are used to provide constant light output with reliability, accuracy, and power efficiency that surpasses incandescent lamps used in the same instruments historically. This invention disclosed in this application is an improved design of a constant-intensity light source that uses an LED. The invention may be used for many applications, but it will be described as implemented in turbidity and/or nephelometry.
Turbidity is an expression of the optical properties of a liquid that causes light rays to be scattered and absorbed rather than transmitted in a straight line through a sample. In other words, turbidity is a measure of muddiness or cloudiness of water. One also speaks of turbidity measurement when characterizing the physical attributes of a colloidal suspension. When a light beam is incident on a colloidal suspension of undissolved finely distributed particles, the suspended particles scatter the light in all directions. The scatter depends on the size of the particles and the spectral properties of the light used to make the measurement. Turbidimeter is a device that is used for quantifying the turbidity of liquids. This measurement is generally performed optically and the units are established based on optical transparency and scattering.
The intensity of scattered light is affected by many variables including wavelength, particle size, color, and shape. The water treatment authority considers all particles of less than 0.45 microns in diameter as being dissolved, but particles smaller than 0.45 microns will also scatter light. The amount of scattered light is not the same in all directions and the spatial distribution pattern varies with particle size. Scattering distribution patterns show that when particles are equal to or larger than the wavelength of the incident light beam, there is a higher amount of forward scattered light, but as the particle size becomes smaller the scattering in all directions prevails so that particles smaller than 0.05 microns in diameter (colloids) scatter light equally in all directions. Other factors that influence light scattering are particle color, particle shape, difference between the refractive indexes of the particle and the sample fluid.
Turbidity measurements are performed on a vial containing the liquid under test or across a transparent pipe through which the liquid under test flows. FIG. 1(a) shows schematically the block diagram of optical measurements that may be performed during a turbidity test. The liquid under test is located or flows inside the pipe (or vial) 101 with transparent walls. A collimated light beam 102 is emitted from a light source 103, directed through the pipe 101 and is detected at a transmitted-light detector 104. Comparing the intensity of the light 103 emitted from the source and the intensity of the light detected by the transmitted light detector 104 gives information about the absorptance of the liquid in the pipe 101. The absorption may be a result of light scattering out of the collimated beam in which case this light can be detected at a right angle away from the original beam direction, in the 90-degree detector, also referred to as the nephelometric detector. When light-scattering in the sample is weak, the reduction of the intensity detected by the transmitted-light detector 104 is small relative to the starting beam intensity 102 and often becomes comparable to the stability drift of the optical source. The stability of the optical source hence directly limits the ability of the turbidimeter to quantify very low turbidity values. In this situation, the 90-degree detector may provide more accurate reading as the in low turbidity samples a small amount of light that scatters sideways has the same stability drift as the original source. However, signal to noise ratio for the nephelometeric detector is worse because the low level light here detected may become comparable to the stray light scattered around the measurement unit and the optics. Further measurement positions, shown in FIG. 1A, can be used to improve the accuracy: Forward scattering 106 and backward scattering 107 of light from the sample holder.
A turbidity measurement basic instrument design uses a single light source and a single photodetector located at 90 degrees to the transmitted light as shown in FIG. 1a with light path 103-101-105. Although very simple, this design has the inherent problem in that the stability of the light source 103 (due to ageing and temperature changes) directly degrades the accuracy of the reading. Repeated calibrations are needed to maintain accuracy. Using ratios of multiple detectors and multiple sources increases the accuracy and stability of measured turbidity values. Some of the practical methods are the Dual-Beam Method (DBM) and Modulated Four-Beam Method (MFBM). The DBM uses a single light source which is split by an oscillating minor into a measuring beam and a reference beam. The measurement is made differentially with a single photodetector detecting the different light intensities of both beams. This method reduces the need for frequent calibration and, when used with a monochromatic light source, totally eliminates the need for calibration. The accuracy is maintained if all the optical components in the system (including the mirror that switches the beam direction) are stable with temperature and ageing (reflections/refractions in the light path). Still the only value measured is the ratio of the transmitted to scattered light intensity. The MFBM uses two light sources and two photodetectors spaced at 90° intervals around a circular sample chamber. Sequentially, the sensor accomplishes two measurements. The two light sources light up alternatively, and when each of them is lit, two of the detectors (one straight forward and one at 90-degrees as detectors 104 and 105 in FIG. 1a, respectively) measure light intensity. These four measurements are sufficient to evaluate the ratio between the transmitted to scattered light intensities without the need for calibration and independent of individual conversion efficiencies of the detectors and the light-sources. Even with four measurements, the only quantity obtained is the ratio of transmitted to scattered light.
Although turbidity measurement technology is well developed and commercialized there still are specific applications for which improvements are needed. First, the complete evaluation of optical scattering properties of the fluid requires determining the transmitted and the scattered light independently. Secondly, reducing power consumption by using only one light source and no microprocessor to handle data analysis enables battery powering which is essential for portable use. Both of these improvements are needed without loss of accuracy and with no repeated calibration.
This application discloses a modification of standard optical sources used in turbidimeters (incandescent lamps and light-emitting diodes) which results in an improvement in the light-source stability against temperature drift and ageing. This improvement enables more accurate nephelometric measurements in all configurations presented.
A typical closed-loop control of the light-output from a light source such as incandescent lamp, light-emitting diode, or laser is illustrated in FIG. 1B. A light source 201 is powered by control electronic 206 and emits a collimated light beam 205 onto a beam splitter 202. The transmitted portion of the beam 203 is useful light to be used for measurement and sensing. The reflected portion 204 of the incident light is captured by the photodetector 207. The intensity of the reflected beam 204 is converted to electric current 205 in the photodetector 207. The control electronics 206 compares the reflected beam intensity in form of current 205 against a reference 208.
The primary factors producing intensity drift in the output beam intensity versus temperature are (a) temperature drift in the reference 208, (b) the change in the emission wavelength or emission spectra of the light source so that the transmittance to reflectivity ratio of the beam-splitter changes with temperature, and (c) scattered light reaching the detector and offsetting the measured power is also temperature dependent.
An important factor coming to play when the optical source is a single-mode laser that emits monochromatic and coherent output beam: Interference fringes appearing on transparent objects external to the source. If the optical source 103 in FIG. 1a is a single-mode laser with a coherent beam, the passage of the light through the walls of the vias or the pipe 101 containing a liquid will produce interference fringes—modulation in transmission that changes with wavelength and hence indirectly with temperature. The interference fringes produce loss of correlation between the different reflections/transmission measurements of the light beam 102 and degrade measurement sensitivity. If interference is a problem in the measurement setup, as it is in turbidity measurement, there is an advantage in using incoherent optical sources. Light-emitting diodes have coherence length which is significantly smaller than the thickness of the walls on most glass pipes and test vials. For this reason, interference fringes rarely occur in the measurement.